History

Although they occur naturally in many foods, the flavor contributions made by glutamic acid and other amino acids were only scientifically identified early in the twentieth century. The substance was discovered and identified in the year 1866, by the German chemist Karl Heinrich Ritthausen who treated wheat gluten (for which it was named) with sulfuric acid.[5] In 1908 Japanese researcher Kikunae Ikeda of the Tokyo Imperial University identified brown crystals left behind after the evaporation of a large amount of kombu broth as glutamic acid. These crystals, when tasted, reproduced the ineffable but undeniable flavor he detected in many foods, most especially in seaweed. Professor Ikeda termed this flavor umami. He then patented a method of mass-producing a crystalline salt of glutamic acid, monosodium glutamate.[6][7]

Function and uses

Metabolism

Glutamate is a key compound in cellular metabolism. In humans, dietary proteins are broken down by digestion into amino acids, which serve as metabolic fuel for other functional roles in the body. A key process in amino acid degradation is transamination, in which the amino group of an amino acid is transferred to an α-ketoacid, typically catalysed by a transaminase. The reaction can be generalised as such:

R1-amino acid + R2-α-ketoacid ⇌ R1-α-ketoacid + R2-amino acid

A very common α-keto acid is α-ketoglutarate, an intermediate in the citric acid cycle. Transamination of α-ketoglutarate gives glutamate. The resulting α-ketoacid product is often a useful one as well, which can contribute as fuel or as a substrate for further metabolism processes. Examples are as follows:

Ammonia (as ammonium) is then excreted predominantly as urea, synthesised in the liver. Transamination can thus be linked to deamination, effectively allowing nitrogen from the amine groups of amino acids to be removed, via glutamate as an intermediate, and finally excreted from the body in the form of urea.

Glutamate is also a neurotransmitter (see below), which makes it one of the most abundant molecules in the brain. Malignant brain tumors known as glioma or glioblastoma exploit this phenomenon by using glutamate as an energy source, especially when these mutations become more dependent on glutamate due to mutations in the gene IDH1.[9][10]

Glutamic acid has been implicated in epileptic seizures. Microinjection of glutamic acid into neurons produces spontaneous depolarisations around one second apart, and this firing pattern is similar to what is known as paroxysmal depolarizing shift in epileptic attacks. This change in the resting membrane potential at seizure foci could cause spontaneous opening of voltage-activated calcium channels, leading to glutamic acid release and further depolarization .

Experimental techniques to detect glutamate in intact cells include using a genetically engineered nanosensor.[20] The sensor is a fusion of a glutamate-binding protein and two fluorescent proteins. When glutamate binds, the fluorescence of the sensor under ultraviolet light changes by resonance between the two fluorophores. Introduction of the nanosensor into cells enables optical detection of the glutamate concentration. Synthetic analogs of glutamic acid that can be activated by ultraviolet light and two-photon excitation microscopy have also been described.[21] This method of rapidly uncaging by photostimulation is useful for mapping the connections between neurons, and understanding synapse function.

Evolution of glutamate receptors is entirely the opposite in invertebrates, in particular, arthropods and nematodes, where glutamate stimulates glutamate-gated chloride channels. The beta subunits of the receptor respond with very high affinity to glutamate and glycine.[22] Targeting these receptors has been the therapeutic goal of anthelmintic therapy using avermectins. Avermectins target the alpha-subunit of glutamate-gated chloride channels with high affinity.[23] These receptors have also been described in arthropods, such as Drosophila melanogaster[24] and Lepeophtheirus salmonis.[25] Irreversible activation of these receptors with avermectins results in hyperpolarization at synapses and neuromuscular junctions resulting in flaccid paralysis and death of nematodes and arthropods.

L-Glutamate at physiological conditions

Brain nonsynaptic glutamatergic signaling circuits

Extracellular glutamate in Drosophila brains has been found to regulate postsynaptic glutamate receptor clustering, via a process involving receptor desensitization.[26] A gene expressed in glial cells actively transports glutamate into the extracellular space,[26] while, in the nucleus accumbens-stimulating group II metabotropic glutamate receptors, this gene was found to reduce extracellular glutamate levels.[27] This raises the possibility that this extracellular glutamate plays an "endocrine-like" role as part of a larger homeostatic system.

Stiff-man syndrome is a neurologic disorder caused by anti-GAD antibodies, leading to a decrease in GABA synthesis and, therefore, impaired motor function such as muscle stiffness and spasm. Since the pancreas has abundant GAD, a direct immunological destruction occurs in the pancreas and the patients will have diabetes mellitus.

Flavor enhancer

Glutamic acid, being a constituent of protein, is present in every food that contains protein, but it can only be tasted when it is present in an unbound form. Significant amounts of free glutamic acid are present in a wide variety of foods, including cheese and soy sauce, and is responsible for umami, one of the five basic tastes of the human sense of taste. Glutamic acid is often used as a food additive and flavor enhancer in the form of its salt, known as monosodium glutamate (MSG).

Nutrient

All meats, poultry, fish, eggs, dairy products, and kombu are excellent sources of glutamic acid. Some protein-rich plant foods also serve as sources. 30% to 35% of the protein in wheat is glutamic acid. Ninety-five percent of the dietary glutamate is metabolized by intestinal cells in a first pass.[28]

Production

China-based Fufeng Group Limited is the largest producer of glutamic acid in the world, with capacity increasing to 300,000 tons at the end of 2006 from 180,000 tons during 2006, putting them at 25%–30% of the Chinese market. Meihua is the second-largest Chinese producer. Together, the top-five producers have roughly 50% share in China. Chinese demand is roughly 1.1 million tons per year, while global demand, including China, is 1.7 million tons per year.

^Meldrum, B. S. (2000). "Glutamate as a neurotransmitter in the brain: Review of physiology and pathology". The Journal of nutrition130 (4S Suppl): 1007S–1015S.

^Meldrum, B. S. 2000. Glutamate as a neurotransmitter in the brain: review of physiology and pathology. The Journal of Nutrition. [Online] 130. Available at:http://jn.nutrition.org/content/130/4/1007.full.pdf Accessed: 22/10/14

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